Fix bti support (#830)

Fix bti support (#830)

* bti: add identifier to ffi_closure_SYSV_V_alt

This was missing BTI_C identifier.

Old Code:
ffi_closure_SYSV_V_alt:
0000fffff7f70500:   ldr     x17, [sp, #8]

Signed-off-by: Bill Roberts <bill.roberts@arm.com>
* testsuite: fix whitespace in Makefile.am

Signed-off-by: Bill Roberts <bill.roberts@arm.com>
* aarch64: correct comment describing BTI

The comment is incorrect, BTI is enabled per mapping via mprotect with
PROT_BTI flag set, not per-process. When the loader loads the library,
if the GNU Notes section is missing this, PROT_BTI will not be enabled
for that mapping, but is independent of other mappings.

Signed-off-by: Bill Roberts <bill.roberts@arm.com>
---------

Signed-off-by: Bill Roberts <bill.roberts@arm.com>