Buggy code to use inline assembler for system calls

The code was reused from Embedded 2013. It build failed. To be traced

Errors:
arm-none-eabi/lib/thumb2/libc.a(lib_a-signalr.o): In function `_kill_r':
signalr.c:(.text+0x14): undefined reference to `_kill'
arm-none-eabi/lib/thumb2/libc.a(lib_a-signalr.o): In function `_getpid_r':
signalr.c:(.text+0x28): undefined reference to `_getpid'
arm-none-eabi/lib/thumb2/libc.a(lib_a-sbrkr.o): In function `_sbrk_r':
sbrkr.c:(.text+0x12): undefined reference to `_sbrk'
collect2: ld returned 1 exit status
make: *** [main.bin] Error 1