libgo patch committed: Tweak can-recover heuristic for SPARC

[Ian Lance Taylor]

The gccgo implementation of panic/recover uses the return address of a
function which called recover in order to see if that function was
invoke directly by defer.  It does this by passing the address of a
label which immediately follows the function call.  This requires a
small heuristic, because there may be some machine instructions between
the function call and the label, even though they are adjacent in
GIMPLE.  On SPARC I needed to tweak that heuristic further, because on
SPARC __builtin_return_address returns the address of the function call
instruction, which is at least 8 bytes before the label.  This patch
implements that tweak.  Bootstrapped and ran Go testsuite on
x86_64-unknown-linux-gnu and sparc-sun-solaris2.11 (Solaris has other
unrelated failures).  Committed to mainline.

Ian

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