stage2: progress towards ability to compile compiler-rt

* prepare compiler-rt to support being compiled by stage2
   - put in a few minor workarounds that will be removed later, such as
     using `builtin.stage2_arch` rather than `builtin.cpu.arch`.
   - only try to export a few symbols for now - we'll move more symbols
     over to the "working in stage2" section as they become functional
     and gain test coverage.
   - use `inline fn` at function declarations rather than `@call` with an
     always_inline modifier at the callsites, to avoid depending on the
     anonymous array literal syntax language feature (for now).
 * AIR: replace floatcast instruction with fptrunc and fpext for
   shortening and widening floating point values, respectively.
 * Introduce a new ZIR instruction, `export_value`, which implements
   `@export` for the case when the thing to be exported is a local
   comptime value that points to a function.
   - AstGen: fix `@export` not properly reporting ambiguous decl
     references.
 * Sema: handle ExportOptions linkage. The value is now available to all
   backends.
   - Implement setting global linkage as appropriate in the LLVM
     backend. I did not yet inspect the LLVM IR, so this still needs to
     be audited. There is already a pending task to make sure the alias
     stuff is working as intended, and this is related.
   - Sema almost handles section, just a tiny bit more code is needed in
     `resolveExportOptions`.
 * Sema: implement float widening and shortening for both `@floatCast`
   and float coercion.
   - Implement the LLVM backend code for this as well.

src/stage1/codegen.cpp

@@ -9325,6 +9325,7 @@ Buf *codegen_generate_builtin_source(CodeGen *g) {
     buf_appendf(contents, "pub const single_threaded = %s;\n", bool_to_str(g->is_single_threaded));
     buf_appendf(contents, "pub const abi = std.Target.Abi.%s;\n", cur_abi);
     buf_appendf(contents, "pub const cpu = std.Target.Cpu.baseline(.%s);\n", cur_arch);
+    buf_appendf(contents, "pub const stage2_arch: std.Target.Cpu.Arch = .%s;\n", cur_arch);
     buf_appendf(contents, "pub const os = std.Target.Os.Tag.defaultVersionRange(.%s);\n", cur_os);
     buf_appendf(contents,
         "pub const target = std.Target{\n"