Remove needs-asm-support directive in tests with explicit targets

The `needs-asm-support` directive checks whether the host architecture
supports inline assembly, not the target architecture. For tests that
explicitly specify a target via `--target` in their compile-flags, this
directive is incorrect and unnecessary.

These tests are cross-compiling to specific targets (like x86_64, arm,
aarch64, riscv, etc.) that are already known to have stable asm support.
The directive was causing these tests to be incorrectly skipped on hosts
that don't support asm, even though the target does.

Tests with explicit targets should rely on `needs-llvm-components` to
ensure the appropriate backend is available, rather than checking host
asm support.

Improve documentation about `needs-asm-support` directive.

Author: Jesus Checa Hidalgo

src/tests/directives.md

@@ -163,8 +163,10 @@ Some examples of `X` in `ignore-X` or `only-X`:
 The following directives will check rustc build settings and target
 settings:
 
-- `needs-asm-support` — ignores if it is running on a target that doesn't have
-  stable support for `asm!`
+- `needs-asm-support` — ignores if the **host** architecture doesn't have
+  stable support for `asm!`. For tests that cross-compile to explicit targets
+  via `--target`, use `needs-llvm-components` instead to ensure the appropriate
+  backend is available.
 - `needs-profiler-runtime` — ignores the test if the profiler runtime was not
   enabled for the target
   (`build.profiler = true` in rustc's `bootstrap.toml`)