cmd/link: resolve magic value in gdbscript section generation

According to the .debug_gdb_scripts section specification
[https://sourceware.org/gdb/onlinedocs/gdb/dotdebug_005fgdb_005fscripts-section.html],
each entry begins with a non-null prefix byte that specifies the kind of entry.
This commit resolves a question about magic byte and replaces a
hardcoded value with a meaningful constant "GdbScriptPythonFileId"
inside writegdbscript function.

Change-Id: I456c742bcb539a5853b9e2a6811033f35c37e7d4
GitHub-Last-Rev: 2f1c4cb9eec582a6e381fa866738aa17278005c2
GitHub-Pull-Request: golang/go#47646
Reviewed-on: https://go-review.googlesource.com/c/go/+/341391


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>