build-sys: use more generic regular expression to generate syscall-list.txt correctly

build-sys: use more generic regular expression to generate syscall-list.txt correctly

Currently MIPS and ARM define syscall numbers for multiple ABI in one
<asm/unistd.h>. The #define statments for each syscall are formated as:

 #define __NR_scname (BASE_OFFSET + sc_number)

Thus we need a more generic regular expression to match these in awk.