Simplify inclusion of stackvma-mincore.c.

* src/stackvma-mincore.c (mincore_get_vma): Renamed from
sigsegv_get_vma.
* src/stackvma-linux.c: No need to define 'sigsegv_get_vma' and 'STATIC'
macros before including stackvma-mincore.c.
* src/stackvma-freebsd.c: Likewise.
* src/stackvma-netbsd.c: Likewise.
* src/stackvma-mquery.c: Likewise.
* src/stackvma-procfs.c: Likewise.
* src/stackvma-aix.c: New file.
* src/Makefile.am (EXTRA_DIST): Add it.
* configure.ac (CFG_STACKVMA): Set to stackvma-aix.c instead of
stackvma-mincore.c.