cmd/link: code cleanup in macho_combine_dwarf.go

- remove global variables
- add missing error checks
- add missing (*os.File).Close calls
- null-check text section correctly
- reduce some variable scopes
- use bytes.Buffer instead of appended slice
- reduce integer casting

Change-Id: Ib9ad41adefed4d61047f53e27046389182eef786
Reviewed-on: https://go-review.googlesource.com/c/go/+/218501


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>