Update configure machinery to detect PTRACE_GETREGS
Using #ifdef PTRACE_GETREGS gives difficulties as in some
platforms (e.g. mips) PTRACE_GETREGS is not defined as a
preprocessor symbols unless linux/ptrace.h is included.
However, including linux/ptrace.h causes compilation error on some
other platforms

=> better detect that PTRACE_GETREGS can be used using the
configure machinery.

This should make vgdb PTRACE_GETREGS working again on
various platforms (x86/amd64/mips/...) as PTRACE_GETREGS
was disabled on all of these following r13471



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13482 a5019735-40e9-0310-863c-91ae7b9d1cf9
Announce fix for BZ 322851 (VEX r2736).


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13483 a5019735-40e9-0310-863c-91ae7b9d1cf9