compiler: add function and global section pragmas

This patch adds a new pragma for functions and globals to set the
section name. This can be useful to place a function or global in a
special device specific section, for example:

  * Functions may be placed in RAM to make them run faster, or in flash
    (if RAM is the default) to not let them take up RAM.
  * DMA memory may only be placed in a special memory area.
  * Some RAM may be faster than other RAM, and some globals may be
    performance critical thus placing them in this special RAM area can
    help.
  * Some (large) global variables may need to be placed in external RAM,
    which can be done by placing them in a special section.

To use it, you have to place a function or global in a special section,
for example:

    //go:section .externalram
    var externalRAMBuffer [1024]byte

This can then be placed in a special section of the linker script, for
example something like this:

    .bss.extram (NOLOAD) : {
        *(.externalram)
    } > ERAM

transform/globals.go

@@ -11,7 +11,7 @@ import "tinygo.org/x/go-llvm"
 func ApplyFunctionSections(mod llvm.Module) {
 	llvmFn := mod.FirstFunction()
 	for !llvmFn.IsNil() {
-		if !llvmFn.IsDeclaration() {
+		if !llvmFn.IsDeclaration() && llvmFn.Section() == "" {
 			name := llvmFn.Name()
 			llvmFn.SetSection(".text." + name)
 		}