better detect jit cpuarch with possible cmdline overrides

[rurban]

amd64 vs i386:

config::auto::arch just checks the perl5 config for the cpu type, esp. if 32-bit or 64-bit is used.

But it is easily possible to override that on the cmdline, and generate the wrong cpuarch then.

• We could check for known arch overrides in ccflags/ldflags, like -m64, -m32 -Wl,-melf_x86_64 and such.
• Or we could just check the ptr size with cc_build check in the compiler check and adjust the cpuarch later.

e.g. On solaris 10 with a 32-bit perl I use:

--ccflags='-m64 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_TS_ER
RNO' --ldflags='-m64' --linkflags='-m64'
--ccflags='-m64 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_TS_ER
RNO' --ldflags='-m64' --linkflags='-m64' --floatval='long double'