Modern golang.org/x/sys/unix (v0.36+) declares its linknames
detached from function declarations, e.g.:
func syscall_syscall(...)
//go:linkname syscall_syscall syscall.syscall
TinyGo's pragma parser only inspected function doc comments and
therefore missed these, producing link errors like:
undefined symbol: _golang.org/x/sys/unix.syscall_syscall
Extend parsePragmas to also walk the enclosing *ast.File's
free-standing comments for //go:linkname directives matching the
function's name. Function-attached directives still take
precedence. The existing 'unsafe' import gate is preserved.
Fixes#4395, #5365
Evan Wies
35a61ac8c5