runtime/trace: fix tracing of blocking system calls
The placement and invocation of traceGoSysCall when using
entersyscallblock() instead of entersyscall() differs enough that the
TestTraceSymbolize test can fail on some platforms.
This change moves the invocation of traceGoSysCall for entersyscall() so
that the same number of "frames to skip" are present in the trace as when
entersyscallblock() is used ensuring system call traces remain identical
regardless of internal implementation choices.