* configure.ac: Add header check for 'sys/auxv.h'; Add function check
for 'getauxval'.
* src/hwf-arm.c [HAVE_SYS_AUXV_H && HAVE_GETAUXVAL]: Include
'sys/auxv.h'.
(HAS_SYS_AT_HWCAP): Enable AT_HWCAP if have 'getauxval' in addition of
__linux__.
(AT_HWCAP, AT_HWCAP2, HWCAP_NEON, HWCAP2_AES, HWCAP2_PMULL)
(HWCAP2_SHA1, HWCAP2_SHA2, HWCAP_ASIMD, HWCAP_AES)
(HWCAP_PMULL, HWCAP_SHA1, HWCAP_SHA2): Define these macros only if not
already defined.
(get_hwcap) [HAVE_SYS_AUXV_H && HAVE_GETAUXVAL]: Use 'getauxval' to
fetch HW capability flags.