[ELF] Support (TYPE=<value>) to customize the output section type

The current output section type allows to set the ELF section type to
SHT_PROGBITS or SHT_NOLOAD. This patch allows an arbitrary section value
to be specified. Some common SHT_* literal names are supported as well.

```
SECTIONS {
  note (TYPE=SHT_NOTE) : { BYTE(8) *(note) }
  init_array ( TYPE=14 ) : { QUAD(14) }
  fini_array (TYPE = SHT_FINI_ARRAY) : { QUAD(15) }
}
```

When `sh_type` is specified, it is an error if an input section has a different type.

Our syntax is compatible with GNU ld 2.39 (https://sourceware.org/bugzilla/show_bug.cgi?id=28841).

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D118840