Look in the Linker Listing File (default *.m51 for BL51) - commonly known as the "Map file"

Following are some extracts from the Map file of the Keil Measure example - note all the references to the ?C?... library stuff.

You need to look at the map files from both your Keil and your SiLabs builds, and see what they're telling you about the Library stuff...

BL51 BANKED LINKER/LOCATER V5.03                                                        09/26/2003  20:41:06  PAGE 1

BL51 BANKED LINKER/LOCATER V5.03, INVOKED BY:
C:PROGRAM FILESKEILC51BINBL51.EXE Measure.obj, Mcommand.obj, Getline.obj, START900.obj TO measure RAMSIZE (256)

MEMORY MODEL: SMALL WITH FLOATING POINT ARITHMETIC

INPUT MODULES INCLUDED:
  Measure.obj (MEASURE)
  Mcommand.obj (MCOMMAND)
  Getline.obj (GETLINE)
  START900.obj (?C_STARTUP)
  C:PROGRAM FILESKEILC51LIBC51FPS.LIB (?C?FPADD)
  C:PROGRAM FILESKEILC51LIBC51FPS.LIB (?C?FPMUL)
  C:PROGRAM FILESKEILC51LIBC51FPS.LIB (?C?FPCMP)
  C:PROGRAM FILESKEILC51LIBC51FPS.LIB (?C?FCAST)
  C:PROGRAM FILESKEILC51LIBC51FPS.LIB (?C?CASTF)
  C:PROGRAM FILESKEILC51LIBC51FPS.LIB (PRINTF)
  C:PROGRAM FILESKEILC51LIBC51FPS.LIB (SCANF)

and

LINK MAP OF MODULE:  measure (MEASURE)

            TYPE    BASE      LENGTH    RELOCATION   SEGMENT NAME
            -----------------------------------------------------

            * * * * * * *   D A T A   M E M O R Y   * * * * * * *
            REG     0000H     0008H     ABSOLUTE     "REG BANK 0"
            REG     0008H     0008H     ABSOLUTE     "REG BANK 1"
            DATA    0010H     0001H     UNIT         ?DT?GETCHAR
            IDATA   0011H     000FH     UNIT         _IDATA_GROUP_
            BIT     0020H.0   0001H.3   UNIT         _BIT_GROUP_
            BIT     0021H.3   0000H.3   UNIT         ?BI?MEASURE
            BIT     0021H.6   0000H.1   UNIT         ?BI?GETCHAR
                    0021H.7   0000H.1                *** GAP ***
            DATA    0022H     002CH     UNIT         _DATA_GROUP_
            DATA    004EH     0014H     UNIT         ?DT?MEASURE
            IDATA   0062H     0001H     UNIT         ?STACK

            * * * * * * *  X D A T A   M E M O R Y  * * * * * * *
            XDATA   0000H     01FFH     UNIT         ?XD?MEASURE

            * * * * * * *   C O D E   M E M O R Y   * * * * * * *
            CODE    0000H     0003H     ABSOLUTE     
            CODE    0003H     0005H     UNIT         ?PR?GETCHAR?UNGETCHAR
                    0008H     0003H                  *** GAP ***
            CODE    000BH     0003H     ABSOLUTE     
            CODE    000EH     07A3H     UNIT         ?C?LIB_CODE
            CODE    07B1H     046FH     UNIT         ?PR?PRINTF?PRINTF

and

SYMBOL TABLE OF MODULE:  measure (MEASURE)

  VALUE           TYPE          NAME
  ----------------------------------
  -------         MODULE        ?C?FPADD
  C:0015H         PUBLIC        ?C?FPADD
  C:0011H         PUBLIC        ?C?FPSUB
  -------         ENDMOD        ?C?FPADD

  -------         MODULE        ?C?FPMUL
  C:0106H         PUBLIC        ?C?FPMUL
  -------         ENDMOD        ?C?FPMUL

  -------         MODULE        ?C?FPCMP
  C:020EH         PUBLIC        ?C?FPCMP
  C:020CH         PUBLIC        ?C?FPCMP3
  -------         ENDMOD        ?C?FPCMP

  -------         MODULE        ?C?FCAST
  C:0283H         PUBLIC        ?C?FCASTC
  C:027EH         PUBLIC        ?C?FCASTI
  C:0279H         PUBLIC        ?C?FCASTL
  -------         ENDMOD        ?C?FCAST

  -------         MODULE        ?C?CASTF
  C:02B7H         PUBLIC        ?C?CASTF
  -------         ENDMOD        ?C?CASTF

  -------         MODULE        PRINTF
  D:002EH         PUBLIC        ?_PRINTF?BYTE
  D:002EH         PUBLIC        ?_SPRINTF?BYTE
  C:0813H         PUBLIC        _PRINTF
  C:080DH         PUBLIC        _SPRINTF
  -------         ENDMOD        PRINTF