As we don't sort local symbols, don't sort non-local symbols. This makes
non-local symbols appear in their register order, which matches GNU as. The
register order is nice in that you can write tests with interleaved CHECK
prefixes, e.g.
```
// CHECK: something about foo
.globl foo
foo:
// CHECK: something about bar
.globl bar
bar:
```
With the lexicographical order, the user needs to place lexicographical smallest
symbol first or keep CHECK prefixes in one place.
-r --gc-sections is usually not useful because it just makes intermediate output
smaller. https://bugs.llvm.org/show_bug.cgi?id=46700#c7 mentions a use case:
validating the absence of undefined symbols ealier than in the final link.
After D84129 (SHT_GROUP support in -r links), we can support -r
--gc-sections without extra code. So let's allow it.
Reviewed By: grimar, jhenderson
Differential Revision: https://reviews.llvm.org/D84131
Fangrui Song