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

compiler/testdata/pragma.go

@@ -39,3 +39,28 @@ func inlineFunc() {
 //go:noinline
 func noinlineFunc() {
 }
+
+// This function should have the specified section.
+//go:section .special_function_section
+func functionInSection() {
+}
+
+//export exportedFunctionInSection
+//go:section .special_function_section
+func exportedFunctionInSection() {
+}
+
+// This function should not: it's only a declaration and not a definition.
+//go:section .special_function_section
+func undefinedFunctionNotInSection()
+
+//go:section .special_global_section
+var globalInSection uint32
+
+//go:section .special_global_section
+//go:extern undefinedGlobalNotInSection
+var undefinedGlobalNotInSection uint32
+
+//go:align 1024
+//go:section .global_section
+var multipleGlobalPragmas uint32