[buildattr64] Clarify at most one .ARM.attributes section

BuildAttributes have file scope, therefore only a single
ELF BuildAttributes section is needed. While the 32-bit
ABI had no restriction on the number of ELF sections, in
practice there was always one, and creating more than one
was difficult.

By restricting the number of sections to at most one consumers
don't need to account for multiple build attributes sections
in the same file that contradict each other.

Author: smithp35

buildattr64/buildattr64.rst

@@ -688,6 +688,9 @@ An ELF Attributes section uses a processor specific section with details
     | ``.ARM.attributes`` | ``SHT_AARCH64_ATTRIBUTES`` | 0x0   | 1         | 0    | 0          |
     +---------------------+----------------------------+-------+-----------+------+------------+
 
+A relocatable object file can contain at most one ELF Attributes
+section.
+
 In the contents of an ELF attributes section, a consumer may not assume
 the natural alignment of a data type such as ``uint32``.